Ceramic materials have been used in dentistry in order to obtain natural-looking dental restorations such as porcelain fused-to-metal and all-ceramic restorations. Ceramics are highly desirable for this purpose since they can be colored to closely resemble the teeth they must replace, resist degradation inside the oral cavity and remain biocompatible even after years of continuous contact with mammalian tissue.
Typically, porcelain fused-to-metal (PFM) restorations are fabricated by applying a dental porcelain powder in aqueous slurry to a metal alloy framework and firing the porcelain at high temperature to form a tight, impervious porcelain layer having the appearance of natural dentition. Those skilled in the art recognize that it is important that the firing temperature of the porcelain be at least 100.degree. C. below the solidus temperature of the alloy used as the metal framework and that the coefficient of thermal expansion of the porcelain (in the range of room temperature to 450.degree. C.) be only very slightly less than that of the metal so that no stress cracks are produced in the porcelain layer during firing and cooling down.
Today, there is an increasing trend in dentistry toward the use of ceramic cores in lieu of metal alloy frameworks to thus provide all-ceramic dental restorations. Where a ceramic is employed as the core of a dental restoration, any porcelain applied to the ceramic framework must possess a coefficient of thermal expansion which is slightly less than that of the ceramic to avoid production of stress cracks in the porcelain.
Metal alloys and ceramics heretofore employed in the manufacture of dental restorations have typically possessed moderately high coefficients of thermal expansion ranging from about 8.times.10.sup.-6 /.degree.C. to about 14.times.10.sup.-6 /.degree.C. However, alloys and ceramics possessing coefficients of thermal expansion of as high as about 18.times.10.sup.-6 /.degree.C. are increasingly being used.
In commonly assigned, copending U.S. application Ser. No. 08/532,179 filed Sep. 22, 1995, the contents of which are incorporated by reference herein, a dental porcelain composition is described which is amorphous, i.e., single phase, and which possesses a moderately high coefficient of thermal expansion closely matching those of conventional alloys and ceramics heretofore employed in the manufacture of dental restorations. This composition is advantageously applied to such conventional alloys and ceramics to provide an extremely smooth, fused glassy surface on the resulting dental restorations.
However, the coefficient of thermal expansion of the single-phase, amorphous dental porcelain described in U.S. application Ser. No. 08/532,179 is too low to be applied to high expansion alloys and porcelains.
A need exists, therefore, for a dental porcelain composition which can be fused to high expansion alloys and ceramics, i.e., those possessing high coefficients of thermal expansion of as high as about 18.times.10.sup.-6 /.degree.C., to thus provide an extremely smooth surface thereon. An example of a high expansion ceramic is Optec.TM. porcelain available from Jeneric/Pentron, Inc. Wallingord, Conn. Optec.TM. porcelain possesses a high crystalline leucite (K.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2) content wherein the leucite crystallites broadly range in diameter from about 0.5 microns to as high as about 40 microns.
Accordingly, it is an object of the present invention to provide a dental porcelain composition which is especially suitable for the preparation and repair of dental restorations, as well as inlays, onlays, and veneers, in applications involving high expansion alloys and/or ceramics.
It is another object of the present invention to provide a dental porcelain possessing a maturing temperature ranging from about 650.degree. to about 1050.degree. C. and a coefficient of thermal expansion (room temperature to 450.degree. C.) ranging from about 12.times.10.sup.-6 /.degree.C.to about 17.5.times.10.sup.-6 /.degree.C., which is chemically and thermally stable and which provides a smooth, non-abrasive surface when applied to high expansion alloys and ceramics.